When an image is projected onto a curved optical surface, such as a windshield or a helmet visor or is captured through a curved optical surface, such as a camera lens, it is common for the image to become optically distorted. In the past, mathematical algorithms containing sets of polynomials have been used to pre-correct an image before it is projected onto a curved surface and/or through the optical system of a helmet-mounted display (HMD) with predetermined optical distortion producing characteristics. This is done to reduce the observable distortion in the finally viewed image. Traditionally, the HMD optics and the inside surface of a helmet visor, which is used to project images to a pilot, are characterized with a sophisticated test rig, and high-order polynomials (e.g. 10th order) have been used to characterize the shape and, therefore, the correction needed to achieve the desired image. However, these high-order polynomial algorithms are computationally intensive and are difficult to implement in a real time system.
The problems associated with high-order polynomials are indeed not new. Before the advent of computer aided design (CAD) systems, draftsmen were known to attempt to plot polynomials by breaking them up into numerous segments, each segment being approximated by simpler polynomials, and a set of French curves was an essential tool in this endeavor.
But with the advent of modern computing, higher order polynomials could be handled, but they often were too slow and required too much computing resources to be used with military video applications.
Today various tricks are known in the art as ways to overcome the problems associated with dealing with such high-order polynomials. One approach is to precompute the pixel distortion and store the amount in a large table. Memory access speeds and memory cost make this approach prohibitive for some applications.
Also, it has been known that other optical systems, such as camera lenses, can have their images improved by digitally correcting problems, such as a drop in brightness as one moves further from the center of a lens.
The present invention overcomes long-standing problems associated with distortion of optical images, especially when images are being processed at video rates.